The objective of the Program is to describe respiratory mechanisms during the early development of the individual organism. All components of prenatal oxygen supply will be studied: maternal, placental, and fetal. The transistion to lung breathing at birth will also be investigated. Projects include studies of the maternal and fetal responses to maternal exercise, measurements of the hemodynamics of the uterine and umbilical circulations, a description of the exchange characteristics of the placenta, studies of uterine ischemia, and morphometric and functional analyses of the development of the allantoic circulation of the hen's egg. Changes in the respiratory characteristics of blood in the postnatal period, neonatal changes in pulmonary vascular resistance in normal and ischemic lambs and the effects of chronic methadone useage on the respiratory center sensitivity of mother and child will also be studied. Particular attention will be paid to the interrelationships among the steps in respiratory gas transfer, and the capability of each step in response to experimental manipulations.